This is a solution I have found for highlighting the selected row and also the currently selected column. This is done through a file called cust.css. I am giving below the full content of my cust.css file.
The file is stored in:

webapps\formspider\apps\CM_MENU\css directory.

CM_MENU is the name of my application.

There are four steps to follow if you want to use this.

Make a css file with the content given below.

Store the file in this directory: webapps\formspider\apps\CM_MENU\css. Replace CM_MENU with your application name.

Edit your application and enter the name of your css file in *CSS Filename* column.

Clear your web browser cache.

Run your application. Test with different colors. Whenever you change the css file make sure to clear the cache before running your application.

There's no API to change highlight color of grid. It comes from one of the CSS files of Formspider.
Therefore, if you want to change it dynamically, you should load new CSS to document's HEAD and override existing one. And you should call this javascript code via Formspider externalMethods.

e.g.

Below code will change highlight color to Red.
If you create an external method and invoke it with parameter like above, you can change the highlight color of grid.